Apparatus for controlling light distribution in line scan optical imaging systems

ABSTRACT

Disclosed is a linear strip mask for controlling image plane light level distribution in line scan optical imaging systems. The mask is fabricated photographically on a strip of emulsion coated mylar. The photographic image which is recorded on the mylar strip is a pattern of alternating black and clear sections, the pattern of black sections chosen to modify the non-uniform light pattern output by the system&#39;s illumination source to obtain uniform image plane illumination. Also disclosed is a line scan optical imaging system utilizing strip masks to compensate for the non-uniform light output along the length of two fluorescent tubes which illuminate the document scan line and the cos 4  drop-off of the imaging system&#39;s lens. Light reflected off the document scan line is transmitted via a four bounce folded mirror system and is received by the lens system. Behind the lens system is positioned a multielement photoelectric sensor for converting light reflected from each unit area of the document line being scanned into electrical signals.

BACKGROUND OF THE INVENTION

The present invention relates in general to electro-optical documentreading systems such as facsimile equipment. More particularly, thepresent invention relates to a mask for controlling image plane lightlevel distribution in line scan optical imaging systems. Still moreparticularly, the present invention relates to a linear strip mask forcontrolling the illumination of a line of text to be read by a line scanoptical imaging system.

In many electro-optical document reading devices, light is directed ontoa line of a printed document, and light is reflected from each elementalarea of the line of the printed document in accordance with the color orblackness of the elemental area. The reflected light is fed through anoptical system, usually including a spherical lens system, to electricalapparatus in which the reflected light is converted to electricalsignals which are used to reproduce the document at a remote location.Typical of such an electrical apparatus is a photoelectric sensor,positioned behind the lens, which converts light reflected from eachunit area of a line of print on the document to electrical signals. Onesuitable photoelectric device comprises an integrated circuit chiphaving a large number of tiny photosensitive elements arranged in aline, each element receiving light from a unit area of each line ofprint.

In the prior art, various light sources have been utilized to illuminatethe line of text being scanned. Thus, in U.S. Pat. No. 4,220,978, anelectro-optical document reader similar to that previously discussedteaches that either incandescent lamps, fluorescent lamps orlight-emitting diodes may be utilized as a source of illumination.

In all of the prior art systems, means must be included to provide asubstantially uniform light distribution at the image plane of theelectrical apparatus which converts the reflected light to electricalsignals. To provide this uniform distribution at the image plane of theelectrical apparatus, a light distribution at the object plane (ordocument) that is brighter at the edges must be provided in order tocompensate for the cos⁴ drop-off of the lens system. Further, in asystem employing a fluorescent tube to illuminate a line of text, thelight intensity output along the length of the tube is roughly of asinusoidal pattern. That is, more light is output at the center of thetube's length than at its ends. Thus, if a line of printed text werepositioned parallel to the fluorescent tube, more light would hit thewords at the center of the line than the words at either end of theline. As a consequence, assuming the black/white content was identicalalong the entire line of text, more light will be reflected off pointsat the center of the line of text than from points at the ends of theline of text. Since the electrical apparatus detects black and whiteareas along the scan line based on the amount of reflected light, theuse of an uncorrected fluorescent tube output to illuminate a scan linewould require that the light sensitivity of each element in theelectrical apparatus be tailored to its position with respect to theline of text. Since such customization would be quite expensive toimplement, various techniques have been proposed to provide asubstantially uniform light distribution at the input to the electricalconversion apparatus.

In one embodiment of the system taught in U.S. Pat. No. 4,220,978, twoincandescent lamps are disposed close to the rear opening of a lightguide which directs light toward the document. The lamps are spacedapart so that some of the light rays from the lamps strike the documentdirectly and some strike the document through reflections from the sidewalls and the top and bottom plates of the light guide, so that thetotal light which reaches the document has the desired distribution oflight intensity across the document; viz., it is brighter at thedocument edges. In this embodiment, light reflected from the document isfed back through the light guide, then through a lens and onto aphotoelectric sensor. The disadvantages of this system is in the factthat a light guide is required, increasing the system's cost. Inaddition, the system is relatively large due to the required length ofthe light guide.

In order to do away with the need for a light guide, various systemsutilizing a fluorescent tube have been suggested. In all known prior artsystems utilizing a fluorescent light source, the illumination subsystemand imaging subsystem are not in the same plane as the most direct pathfrom the light source to the document. Therefore, there are two lightpaths in these systems which may be modified in order to obtain auniform light distribution at the image plane. Thus, a first option isto modify the light output from the fluorescent tube before it reachesthe document. The second option is to modify the reflected light patternbetween the document and lens or electrical detection apparatus.

In U.S. Pat. No. 4,220,978, a system is disclosed wherein a singlefluorescent bulb is suitably masked by an opaque coating to allowgreater light output at its ends than at its center. By using the opaquecoating, the light intensity at a distance from the bulb has theappropriate distribution on the scan line of the document. In thissystem, the bulb is positioned next to a light guide which directs thelight to the document. Light reflected from the document does not returnthrough the light guide, but travels along a path disposed outside thelight guide. Thus, the light guide is disposed at such an angle to thedocument that light is reflected from the document on an axis which isdisposed above the light guide to a suitable optical apparatus andelectro-optical pickup mechanism. Although this system solves theproblem of providing the appropriate light intensity along the scanline, it requires that the fluorescent tubes be suitably coated with theopaque mask. In addition, this system requires the use of a light guidewhich enlarges the size of the system and adds to its cost.

Systems utilizing a fluorescent tube, but not requiring a light guide,are also known in the prior art. In such systems, the fluorescent tubeis positioned parallel to and above the line of text to be scanned, andin close proximity to the document. The light from the tube thus travelsdown at an acute angle to illuminate the scan line. A lens is positioneda distance from the document at the same height as the scan line. Thelens focuses the reflected light onto an array of CCD sensors positionedbehind the lens. In this system, the nonlinear light intensity output ofthe fluorescent tube is compensated for by one of two methods.

In the first method, a line aperture is used which has an opening whichvaries in size across the length of the aperture. Thus, the apertureopening is narrow at the center and widens as one moves towards the endsof the aperture. This aperture is positioned in the reflected light pathbetween the scan line on tne document and the lens. This variable sizeaperture thus controls the amount of light entering the lens from eachpoint in the object plane (viz., the document scan line).

The first method suffers from two major drawbacks. The aperture must beaccurately positioned along the axis normal to the scan line axis. Ifnot positioned correctly, the light level will not be as desired. Thesecond drawback is the reduction of modulation transfer of the imagingsystem due to the effect of an obscured lens aperture.

The second method utilizes a photographically produced continuous tonegray-scale gradient filter which varies in density across the length ofthe filter. As in the first method, the line filter is positionedbetween the lens and the object plane where it controls the amount oflight entering the lens from each point on the object plane.

The second method also suffers from two major drawbacks. The varyingdensity filter creates a reduction in the modulation transfer of theimaging system due to the scattering of light by the photographicemulsion. The second major drawback is the excessive cost of continuoustone gray-scale gradient filters.

It is the general object of the present invention to overcome theabove-mentioned drawbacks of the prior art by providing an improvedapparatus for controlling the image plane light level distribution inline scan optical imaging systems.

It is another object of the present invention to provide a repeatableand low cost apparatus for controlling the light level distribution inline scan optical imaging systems.

It is yet another object of the present invention to provide a mask formodifying the light intensity output from a light source.

It is a further object of the present invention to provide a fluorescenttube mask which compensates for the non-uniform light output along thelength of a fluorescent tube and which need not be accurately aligned inthe axis normal to the scan line axis.

It is still another object of the present invention to provide a maskfor controlling light level distribution in line scan optical imagingsystems which does not reduce imaging system modulation transfer.

It is still a further object of the present invention to provide a lowcost and compact system for controlling light level distribution in linescan imaging systems.

It is yet another object of the present invention to provide afluorescent tube mask which compensates for the cos⁴ drop-off ofreflected light prior to transmission through the lens of a line scanoptical imaging system.

These and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription of the preferred embodiment when read in conjunction withthe drawings.

SUMMARY OF THE INVENTION

According to the invention, a linear strip mask is provided forcontrolling image plane light level distribution in line scan opticalimaging systems.

In the preferred embodiment, the mask is fabricated photographically ona strip of emulsion coated mylar. The photographic image which isrecorded on the mylar strip is a pattern of alternating black and clearsections. The pattern of black sections is chosen to provide the desiredlight level distribution for the imaging system.

Although the preferred embodiment utilizes photographic techniques toproduce the mask pattern on the clear mylar, those skilled in the artwill appreciate that various printing or other techniques may beutilized to apply the desired pattern of black lines 42 on the clearmylar. Thus, there is no optical reason why the desired pattern of lines42 could not be printed on the mylar or another type of clear or tintedsubstrate using impact, silkscreen, lithographic, laser, or otherprinting techniques.

In the preferred embodiment, the mask is utilized to produce a uniformlyilluminated image plane in a line scan optical imaging system utilizingfluorescent lamps to illuminate the system. In this application, themask is fabricated with a greater density of black lines (or sections)in the center regions and a decreasing density of black lines as theedges of the mask are approached. The effect of the mask is to restrictthe light level in the center of the object field. Since the intensityof the light output by the fluorescent tubes is greatest at the centerof the tubes and decreases as the ends of the tube are approached, themask serves to compensate for the non-uniform light output along thelength of the tubes so that a light level distribution is obtained onthe non-illuminated side of the mask that is brighter at the edges.Then, as the illumination line passes through the lens system, there isa further brightness drop-off at the edges due to the cos⁴ drop-off As aresult, a uniform image plane illumination line is obtained at thesystem's electrical detection apparatus.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a line scan optical imaging system utilizingthe mask to control light level distribution.

FIG. 2 is a block diagram illustrating typical elements for convertingreflected light received by an image sensor to a picture signal.

FIG. 3 roughly illustrates the light intensity output along the lengthof a fluorescent tube.

FIG. 4 is a front plan view showing a portion of of the mask used tocontrol fluorescent light level distribution in the line scan opticalimaging system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 illustrates the general environment that the present inventionfunctions in. The system 10 includes a motor (not shown) which drivesfeed rollers 12 to incrementally advance the document 14 being read inthe direction shown. The motor may be controlled to advance the documentone scan line position at a time or may advance the documentcontinuously. In the latter mentioned case, reading takes place on thefly.

A lamp circuit (not shown) supplies power to fluorescent tubes 16. Inthe preferred embodiment, Sylvania sixty degree aperture fluorescenttubes are utilized. These tubes only have a phosphor coating on threehundred degrees of their inner surface. As a result, internallygenerated light is emitted from the remaining non-coated surface of thetubes.

Those skilled in the art will appreciate that standard fluorescent tubescould be utilized instead of the sixty degree aperture type. In suchcase, a housing may be provided around a portion of each tube 16 to onlyallow light transmitted from a portion of each tube's outer surface toreach the document.

As shown in FIG. 1, the tubes 16 are positioned so that light exitingthe aperture of the tubes 16 will illuminate the document 14 line beingscanned.

The construction of the masks 18 will be discussed in detail below. Forthe moment, suffice it to say that the masks 18 compensate for thenon-uniform light output along the length of the fluorescent tubes 16such that the entire width of the document 14 scan line is illuminatedwith the desired distribution. Further, the masks 18 compensate for thecos⁴ drop-off as the reflected light passes through the imaging lens 28.

Those skilled in the art will appreciate that although the presentsystem simultaneously scans the entire width of the document 14, only asmall vertical portion of each line of text is scanned in one scanoperation. Thus, the document 14 must be vertically incremented toposition it for each scan operation. Typically, the vertical distancescanned is approximately five thousandths (0.005) an inch for each scanline.

The light from the fluorescent tubes 16 illuminates the document 14 scanline and is reflected off the document 14 in proportion to theblack/white content of the document 14 line being scanned. Lightreflected off the document 14 scan line follows the path indicated bybroken lines. Thus, the reflected light hits the flat surface 20 ofmirror 22, is bounced onto the flat surface 24 of mirror 26, returnsagain to the surface 20 of mirror 22, is reflected back to the surface24 of mirror 26, and then travels along the axis indicated and isreceived by lens system 28. In the preferred embodiment, lens system 28is a spherical, aberration-free lens system 28 mounted to receive lightreflected from the document 14 scan line via mirrors 22 and 26.

Mirrors 22 and 26 are mounted to frame 30 via adhesive 32. The mirrors22,26 are properly aligned to implement the required folded reflectionpath using the apparatus described in pending U.S. patent applicationSer. No. 548,553, filed Nov. 3, 1983.

Behind the lens system 28 is positioned a photoelectric sensor 34 forconverting light reflected from each unit area of the document 14 linebeing scanned to electrical signals. In the preferred embodiment, thephotoelectric sensor 34 is a CCD image sensor 34 having a large numberof photoelectric conversion elements arranged in a line, each elementreceiving reflected light from a unit area of each document 14 scanline.

FIG. 2 shows typical elements utilized to convert the reflected lightreceived by image sensor 34 to a picture signal. The CCD image sensor 34utilizes a plurality of clock signals produced in a control section (notshown). The sensor drivers 36 convert these clock signals into voltagesappropriate to the sensor 34. The AGC circuit 38 maintains the picturesignal output by the sensor 34 nearly constant against differences incolor and density of the documents 14 and fluctuations in the brightnessof the fluorescent tubes 16. It assumes areas reflecting the light mostto be white and operates so that outputs for these areas are constant.

The CCD image sensor 34 outputs analog signals proportional to inputlight intensity. The digitizer 40 converts the analog signals intobinary signals, "1" for white and "0" for black.

It should be noted that the description of the conversion subsystem ofFIG. 2 is merely illustrative of one technique for converting lightreflected from the document 14 line being scanned into a digital image.Those skilled in the art will therefore appreciate that many alternativetechniques are available.

As previously discussed, the fluorescent tubes 16 utilized in thepreferred embodiment are standard sixty degree aperture type. FIG. 3roughly illustrates the intensity of light output by the tubes 16 atpoints along their length. As shown, light output is highest at thecenter of each tube's length and falls off towards the ends of the tube16. The pattern of light intensity output by each tube 16 is roughlysinusoidal.

In order to provide the desired illumination along the entire width ofthe document 14 line being scanned, the present invention provides amask 18 (FIG. 4) which compensates for the non-uniform light output ofthe fluorescent tubes 16 and the cos⁴ drop-off of the imaging lens 28.As shown in FIG. 1, the mask 18 is mechanically held in place tangentialto the surface of each tube 16 (by means not shown) and extendscompletely across the aperture and length of each tube 16. Since themask 18 is placed between the illumination source (tubes 16) and theobject plane (document 14) and not between the lens system 28 and theobject plane, the modulation transfer of the system 10 is not affected.

The masks 18 (FIG. 4) are fabricated photographically onto strips ofemulsion coated clear mylar using techniques well known to those ofordinary skill in the photographic arts. The photographic image which isrecorded on the mylar strip is a pattern of alternating black and clearsections, the black sections being in the form of lines 42.

The pattern of black lines 42 on the mylar strip mask 18 is chosen toprovide the desired light level distribution by blocking portions of thelight emitted by each fluorescent tube 16 from reaching the document 14.Those skilled in the art will appreciate that there are several factorswhich must be considered when determining the distribution of the blacklines 42 on the mask 18. These factors include:

The polar pattern of the illumination source (which are fluorescenttubes 16 in the preferred embodiment);

The image plane light level dropoff due to lens system 28; and

The transmissivity of the clear mylar.

Those skilled in the art will also appreciate that these three factorscan be calculated or measured for the particular imaging system. A wellknown series of mathematical calculations can then be used to computethe distribution of black lines 42 needed to produce the desired imageplane distribution. Thus, a first calculation computes the image planelight distribution without the mask 18. A second calculation computesthe object plane light distribution that will be required to produce thedesired final image plane light distribution. A final calculationcomputes the distribution of black lines 42 needed to produce therequired object plane distribution. Those skilled in the art willfurther appreciate that these calculations are dependent on factors suchas the type of illumination source, the type of lens system 28 utilized,the sensitivity of the CCD image sensor 34, the angle of lightreflection and the spacing of the imaging system components.

Due to the complexity of accounting for all of the latter mentionedfactors, empirical methods may be more easily utilized to determine orrefine the distribution of black lines 42 on the masks 18. Thus, in thepreferred embodiment as shown in FIG. 1, an oscilloscope was utilized tomeasure the output of each element of the CCD image sensor 34. An allwhite line was scanned and the spacing of lines 42 adjusted until auniform output from each sensor 34 element was obtained. Those skilledin the art will appreciate that an optimum final pattern of black lines42 is quite easy to obtain and is dependent on the tolerances of thesensor 34 and the detection circuitry (FIG. 2) which receives the outputof the sensor 34.

Once an optimum mask pattern for the using optical system is obtained, amaster mask with the optimum distribution of black lines 42 is drawn.The master is photographically reduced and exposed onto an emulsioncoated mylar sheet. The sheet can then be cut into strips forinstallation into the imaging system 10. FIG. 4 shows in enlarged scalea portion of the mask 18 utilized in the system of FIG. 1. The lateralline 44 in the mask 18 pattern is provided for optically aligning themask 18 in the using optical system 10 and does not appreciably affectthe light distribution reaching the document 14 scan line.

FIG. 1 shows the actual spatial relationship of the elements used in thepreferred embodiment of the present imaging system. In the preferredembodiment of the invention, two fifteen inch long, sixty degreeaperture fluorescent tubes 16 are utilized to illuminate the document 14line being scanned. The mask 18 pattern utilized in the preferredembodiment is shown in FIG. 4. In actuality, the length of the masks 18is the same as the length of each tube 16. Each mask 18 is fabricatedwith a greater frequency of black lines 42 in the center regions, thedensity of black lines 42 decreasing symetrically as the ends of themask 18 are approached. The effect of this mask 18 pattern is torestrict more light transmission at the center of the pattern than atthe ends. As a result, the light intensity reaching the document 14 isbrighter at the edges of the document 14 scan line.

Although the preferred embodiment of the mask 18 is utilized in a systememploying two fluorescent tubes to illuminate the document 14, thoseskilled in the art will appreciate that a similar mask 18 structure maybe utilized with other types of light sources and in systems having oneor more than two light sources. Further, the same type of mask 18 with adifferent pattern of lines 42 may be utilized to obtain any desireddistribution which may be required in other types of imaging,photographic, or, for that matter, any system or apparatus whichrequires controlled illumination.

Having shown and described the preferred embodiment of the presentinvention, those skilled in the art will realize that various omissions,substitutions and changes in forms and details may be made withoutdeparting from the spirit of the invention. It is the intention,therefore, for the scope of the invention to be limited only asindicated in the following claims.

What is claimed is:
 1. A line scan optical imaging system includingmeans for illuminating a line of a printed document and means forconverting light reflected from the line of print to electrical signals,wherein the improvement comprises:mask means, positioned between saidilluminating means and said document, for controlling light transmittedfrom said illuminating means to said printed document line about alongitudinal axis thereof, said mask means having a pattern of opaquemarkings varying longitudinally with respect to the longitudinal axis,said pattern defined by an overlay of a plurality of irregularly spacedlines perpendicular to the longitudinal axis.
 2. The system inaccordance with claim 1 wherein the frequency of the plurality ofirregularly spaced perpendicular lines is proportional to the brightnesslevels of the illuminating means at longitudinal points thereof.
 3. Thesystem in accordance with claim 1 wherein said mask means is positionedadjacent to said illuminating means.
 4. The system in accordance withclaim 1 wherein said illuminating means includes means forsimultaneously illuminating the entire line of said printed document. 5.The system in accordance with claim 1 wherein each one of said pluralityof irregularly spaced perpendicular lines is of equal length.
 6. Thesystem in accordance with claim 5 wherein each one of said plurality ofirregularly spaced perpendicular lines is of equal size.
 7. The imagingsystem in accordance with claim 1 wherein said mask means in a linearstrip mask, said linear strip mask fabricated from an emulsion coatedclear strip of material.
 8. The imaging system in accordance with claim7 wherein said emulsion is photographically processed to record apattern of alternating black and clear sections along the length of theclear strip.
 9. The imaging system in accordance with claim 8 whereinsaid pattern is chosen to block portions of the light from saidilluminating means from reaching the line of print, whereby the line ofprint is illuminated with a selected distribution which is dependent onthe pattern of alternating black and clear sections.
 10. The imagingsystem in accordance with claim 1 wherein said illuminating meansincludes a fluorescent tube light source, said mask means for partiallyblocking light from said fluorescent tube from reaching said line ofsaid printed document.
 11. The imaging system in accordance with claim10 wherein said mask means includes a clear strip of material positionedparallel with the longitudinal axis of said fluorescent tube, sectionsof said strip coated with an opaque coating to form a pattern whichpartially blocks light emanating from the fluorescent tube from reachingthe line of said printed document.
 12. The imaging system in accordancewith claim 11 wherein said pattern includes a plurality of opaqueirregularly spaced apart lines perpendicular to the longitudinal axis ofsaid strip, the frequency of said lines being greatest adjacent topoints along said fluorescent tube where the light output is greatest,the frequency of said lines being lower at points along said fluorescenttube where the light output is least.
 13. The imaging system inaccordance with claim 12 wherein said strip is coated with aphotographic emulsion and said lines are produced by a photographictechnique.
 14. The imaging system in accordance with claim 11 whereinsaid pattern includes a plurality of opaque spaced apart linesperpendicular to the longitudinal axis of said strip, the frequency ofsaid lines being greatest at a point adjacent to the center of saidfluorescent tube and decreasing in frequency as points along the stripadjacent to the ends of the tube are approached.
 15. The imaging systemin accordance with claim 14 wherein said strip is positioned tangentialto the surface of said fluorescent tube.
 16. The imaging system inaccordance with claim 14 wherein said fluorescent tube has an apertureand said mask means covers the entire aperture along the entire lengthof said fluorescent tube.
 17. A mask for controlling image plane lightlevel distribution from a source of illumination, said mask comprising aclear substrate, said substrate coated with a plurality of opaqueparallel lines, the frequency of said parallel lines increasing in areasadjacent to the image plane where it is desired to decrease the amountof light reaching the image plane.
 18. The mask in accordance with claim17 wherein said substrate is coated with a photographic emulsion, andsaid lines are produced by first projecting a pattern corresponding tothe desired line distribution onto said coated strip and then developingsaid emulsion.
 19. The mask in accordance with claim 17 wherein saidsource of illumination is a fluorescent tube and said mask is in theshape of a strip, said plurality of opaque parallel lines runningperpendicular to the longitudinal axis of said strip, said strippositioned parallel to and between said flurorescent tube and said imageplane, the frequency of said parallel lines being greatest in an areaadjacent to the center of said fluorescent tube and decreasing as theends of said strip are approached.
 20. A line scan optical imagingsystem for scanning a line of a document comprising:means forsimultaneously illuminating the entire length of said document line;mirror means for transmitting light reflected from said document line;lens means for receiving a focusing light transmitted by said mirrormeans; photo-optical means for converting said focused light into aplurality of electrical signals; and mask means, positioned between saidilluminating means and said document line, for blocking portions of thelight output from said illuminating means from reaching said documentline, said mask means having a pattern of irregularly spaced parallellines thereon.
 21. The line scan imaging system in accordance with claim20 wherein:said illuminating means includes at least one fluorescenttube; and said mask means is a linear strip mask positioned between saidfluorescent tube and said document, said strip mask including a clearsubstrate coated with said pattern of irregularly spaced parallel lines,said parallel lines being perpendicular to the longitudinal axis of saidsubstrate.
 22. The line scan imaging system in accordance with claim 21wherein the frequency of said parallel lines is greatest at an area nearthe center of said fluorescent tube and decreases in frequency as theends of said linear strip mask are approached.
 23. The line scan imagingsystem in accordance with claim 22 wherein said illuminating meansincludes two of said fluorescent tubes, each of said tubes being of anaperture type and wherein the aperture area of each of said fluorescenttubes has one of said strip masks adjacent to it.
 24. The line scanimaging system in accordance with claim 23 wherein said mirror means isof the folded mirror type and said photo-optical means includes a largenumber of photoelectric conversion elements arranged in a line, eachelement receiving focused light from a unit area of said document line.